... low fee like bittrex with 20k satoshi, and get a fast confirmations on my wallet, i usually wonder how can they do this...
bittrex takes 20K fee from you buy pays 40k fee on the transaction (about 180 s/b)!
go check your tx in a block explorer and you'll see.
Yep, most of my not so recent exchange withdrawals I was charged a withdrawal fee less than the fee that the exchange put on the transaction!